No abstract

Microbial generalists and specialists coexist in the soil environment while having distinctive impacts on microbial community dynamics. In microbial ecology, the underlying mechanisms as to why a species is a generalist or a specialist remain ambiguous. Herein, we collected soils across a national scale and identified bacterial generalists and specialists according to niche breadth at the species level (OTU level), and the single‐nucleotide differences in each species were measured to investigate intraspecific variation (at zero‐radius OTU level). Compared with that of the specialists, the intraspecific variation of the generalists was much higher, which ensured their wider niche breadth and lower variability. The higher asynchrony and different niche preferences of conspecific individuals and the higher dormancy potential within the generalists further contributed to their stability in varying environments. Besides, generalists were less controlled by environmental filtering, which was indicated by the stronger signature of stochastic processes in their assembly, and had higher diversification and transition rates that allowed them to adapt to environmental changes to a greater extent than specialists. Overall, this study provides a new comprehensive understanding of the rules of assembly and the evolutionary roles of bacterial generalists and specialists. It also highlights the importance of intraspecific variation and the dormancy potential in the stability of species.

Fertilization regimes are known to drive succession of the soil microbial community, whereas the assembly rules involved remain elusive. Moreover, the ecological roles of microbial “generalists” and “specialists” in soils with contrasting fertilization regimes have not been characterized. We explored how long-term fertilization regimes (i.e., treatments conducted for at least 30 years) impact the soil bacteria by modifying species richness, diversity, bacterial assembly, and niche breadth. Compared with long-term organic fertilizer input, the soils having undergone chemical-only fertilization contained smaller amounts of carbon resources and had a more acidic environment. This strong environmental constraint lowered the soil bacterial reservoir and resulted in a detectable ecoevolutionary transformation, with both a higher proportion of specialists and a stronger signature of deterministic processes. Overall, this study provided a new comprehensive understanding of the assembly rules of bacterial generalists and specialists under long-term fertilization regimes. This study also highlighted that chemical-only fertilization, a ubiquitous agricultural practice of current conventional agriculture, induced a strong and similar environmental force that transformed the soil microbiota from 28°N to 46°N included in this study. IMPORTANCE Chemical-only fertilization is ubiquitous in contemporary conventional agriculture despite the fact that sustainability of this agricultural practice is increasingly being questioned because of the current observed soil degradation. We explored how long-term chemical-only versus organic-only fertilizations impacted the soil microbiota reservoir in terms of both diversity and induced assembly processes. The results showed that long-term chemical-only fertilization resulted in deep selection pressure on the soil microbial community reservoir, with both a higher proportion of specialists and a stronger signature of deterministic processes. The soil microbiota has clearly changed as a consequence of the fertilization regime. The diagnoses of the functional consequences of these ecoevolutionary changes in relation to agricultural practices are key to imagining agriculture in the time ahead and especially regarding future efforts for the conservation, restoration, and management of the soil microbiota reservoir which is key to the fertility of the ecosystem.